1. Field of the Invention
The present invention relates to a pacemaker of the type which detects the electrical activity of the heart by electrodes disposed in the heart and outputs a pacing pulse from the electrodes, more particularly, relates to a pacemaker which is free from erroneous detection of a pacing signal and after potential etc. as an electrical activity of the heart.
2. Description of the Related Art
At the present time, for example, for patients suffering from acute cardiac infarction, patients after heart surgery, patients suffering from shock, etc., use is generally made of devices known as pacemakers when it is necessary to normalize the heart functions of the patients.
In such patients, use is made of external pacemakers when the need for normalization of the heart functions is transitional, while use is made of the implantation type pacemakers when it is perpetual.
For example, mounting an external pacemaker entails inserting and disposing a catheter with two electrodes in the right ventricle of the heart, then guiding lead wires from the pacing electrodes through the inside of the catheter to the outside of the body, and connecting the pacemaker body to the terminals of the same.
In such an external pacemaker, use is mainly made of a pacemaker of the type referred to as an external pacemaker of the type known as a demand type wherein pacing is performed when the heart beat of the body falls below a certain set number of beats and pacing is not performed with a number of heart beats above the set number of beats. Therefore, the electrical activity of the heart (wave height of 2 to 20 mV) caused between the two electrodes of the catheter inserted into the right ventricle is detected by the pacemaker and when the cycle interval between the R wave of the electrical activity and the next R wave is longer than the time corresponding to the set number of beats, a pacing pulse (wave height of 2 to 4 V and pulse width of 1 to 3 msec) is given between the two electrodes and passes through the cardiac muscle to cause contraction of the cardiac muscle. Therefore, during pacing, the heart is repeatedly made to beat forcibly at the set number of heart beats.
In this pacemaker, the input electrode and the output electrode are the same. Sometimes a weak voltage is received as input and sometimes a high voltage must be output by the same electrode.
Further, the input potential generated between the two electrodes in the right ventricle of the heart is due not only to the electrical activity of the heart. There are also fluctuations due to slow changes, that is, baseline fluctuations. Further, the large residual potential extending over a long period and caused after the pacing pulse is given, known as the after potential, that is, a residual potential as if a large capacitor had been connected in parallel between the electrodes, exists and this potential is input from the input-output terminal of the pacemaker. This residual potential is shown by the symbol A in FIG. 31, while a pseudo load circuit 2 constructed so as to enable pseudo reproduction of the residual potential is shown in FIG. 32. Note that the symbol B in FIG. 31 is a pacing pulse.
Therefore, in a circuit for detecting the electrical activity of the heart, unless the entry of the pacing pulse B and the after potential A into the detection circuit is prevented, these potentials will be detected and discrimination from the electrical activity of the heart will be impossible.
In a circuit for detecting the electrical activity of the heart, first the baseline fluctuation is eliminated using the input side as an AC coupling of a cutoff frequency of 10 to 30 Hz, then the signal is amplified several hundred fold by an amplifier, or the noise of the high band frequency is removed by passing the signal through a low pass filter, to obtain a signal stressing only the R wave of the electrical activity of the heart. This is detected by the comparison circuit when an R wave of more than a predetermined amplitude is input.
Usually, a detection circuit is given the function of detecting the electrical activity of the heart, then stopping the detection for 250 to 300 milliseconds. This is so as to prevent the detection off the S wave, T wave, or premature contraction etc. following the R wave of the electrical activity of the heart as shown in FIG. 33. Therefore, if the pacing pulse etc. is mistakenly detected, there is the inconvenience of the electrical activity of the heart not being able to be detected for the following 250 to 300 milliseconds.
Therefore, to avoid the effect of the pacing pulse and after potential, in the prior art, the following circuits are provided at the input-output terminal of the pacemaker.
First, a first circuit is the circuit shown in FIG. 34. In this circuit, provision is made of a switch 12 which completely separates the input and the output when the electrical activity of the heart is input and when a pacing pulse is output. When this switch 12 is set at the input side, that is, the solid line position of the figure, the electrical activity of the heart from the electrodes passes through the input-output terminal 4 and the switch 12 to reach the amplification circuit 14 where it is amplified and output to a detection circuit for detecting the R wave of the electrical activity of the heart. On the other hand, when outputting the pacing pulse, the switch 12 is set to the output side, that is, the dotted line position of the figure, for the time when the pacing pulse is output. If the input and the output are completely switched in this way, then it is possible to avoid the effects of the pacing pulse.
As such a circuit for performing the same operation as the first circuit, there is also the second circuit shown in FIG. 35. In this circuit, an output stage of the open collector or open drain connection is provided. In the same way as the first circuit, provision is made of a switch 15 which completely separates the input and output when the electrical activity of the heart is input and when the pacing pulse is output.
Further, as a third circuit, there is the circuit shown in FIG. 36. This circuit basically has the same construction as the circuit shown in FIG. 34. The switch 17 illustrated operates in the same way as the switch 12 of FIG. 34. The one different portion is that provision is made, in the input route from the switch 17 to the amplification circuit 14, of an RC filter having a cutoff characteristic of more than 1 Hz comprised of a capacitor 18 and a resistor 19. This filter is provided for eliminating the slowly changing fluctuations, that is, the baseline fluctuations. If the filter is not provided and the input terminal of the amplifier is directly connected to the input-output terminal, there is the danger of saturation of the amplifier due to the baseline fluctuation.
If provision is made of one of the above illustrated three circuits at the input-output terminal of the pacemaker, it is possible to avoid to a certain state the effects of the pacing pulse, but each of the circuits is designed to switch from the output side to the input side right after the output of the pacing pulse, so at the instant of the switching, the residual potential (after potential) is input and, for example, the amplification circuit ends up becoming saturated over a period of several hundred milliseconds and during that time it is impossible to detect the electrical activity of the heart.
To avoid this saturation, it is sufficient to prevent switching of the switch to the input side until the residual potential sufficiently attenuates, but with this the electrical activity of the heart detected by the pacing electrode cannot be input until the switching, so this does not basically resolve the problem.
Therefore, consideration has been given to a circuit which holds the input-output side at a low impedance for a short time after the pacing pulse is detected, forcibly discharges the R wave until a potential of a degree enabling detection of the electrical activity of the heart, then making the input-output side a high impedance and detecting the electrical activity of the heart.
Such a circuit is shown in FIG. 37 and FIG. 38. In the circuit shown in FIG. 37, provision is made of three switches: a switch 21 which connects a resistor 20, for discharging the after potential, to the input-output terminal 4, a switch 22 which turns on for the time when the pacing pulse is output, and a switch 23 for inputting the electrical activity of the heartfrom the input-output terminal 4 through a filter comprised of a capacitor 18 and a resistor 19 to the amplification circuit 14. The function of the filter and the function of the amplification circuit 14 are the same as in the above-mentioned circuit.
If the three switches are operated in the following way, then it is possible to cause saturation of the effect of the after potential. That is, at the time of the output of the pacing pulse, the switch 22 is turned on for about 2 milliseconds and a pulse with a relatively high potential is output to the input-output terminal. At this time, the switch 21 and the switch 23 are turned off. After the output of the pacing pulse, the switch 21 is turned on substantially simultaneously with the turning off of the switch 22 and the residual potential due to the after potential is forcibly attenuated through the resistor 20. This switch 21 is turned on until the residual potential attenuates to an extent not having an effect on the detection of the electrical activity of the heart (several tens of milliseconds).
Note that the switch 23 is turned off at this time too. The after potential sufficiently falls due to the operation of the switch 21, but does not fall completely to 0. Also, there is also baseline fluctuation. Therefore, the input-output terminal often does not become 0. This enables input of the electrical activity of the heart, but the slightly remaining after potential mentioned above is rapidly charged to the capacitor 18, so the amplification circuit 14 receives as input the pulse-like potential shown in FIG. 39(B). When the wave height of the pulse is large, it is not possible to differentiate it from the electrical activity of the heart and erroneous detection results.
To eliminate such trouble, in the circuit of FIG. 38, a switch 24 is connected in parallel with the resistor 19 at the connection point of the capacitor 18 and the resistor 19. This switch 24 is turned on for a period longer than a time constant determined by the capacitor 18 and the resistor 19 just before the switch 21 is turned off. If this is done, then it is possible to eliminate the trouble of receiving the pulse-like potential shown in FIG. 39(B). However, it is necessary to extremely precisely control the turning on and off of the four switches, so the control becomes complicated and therefore a large number of electronic circuits (electronic components) is needed, the device becomes larger, the reliability falls, and the current consumption increases, so there is the trouble of a shorter battery life.